As shown in FIG. 2, a conventional optical attenuator comprises an optical system enclosed in a case 23 which is constructed with optical elements such as spherical lenses an an ND filter. Since the conventional optical attenuator has to be constructed with a combination of many optical elements, it was difficult heretofore to reduce the cost and the size of the system. Further, high manufacturing and adjustment accuracy is required for the construction of the optical system. Moreover, since the optical elements have to be cemented or mechanically bound to set up the optical system, the prior type of system not has not highly reliable from the view point of temperature characteristics and shock-resistance.